tDCS is generally considered safe and comfortable, however isolated reports on skin irritation have suggested local scalp temperature increases as a probable cause. We have developed novel technology for tDCS that improves spatial focality at the cost of increased stimulation electrode current density; high denisty tDCS (HD-tDCS). The goal of this study was to provide information on the thermal effects of tDCS using a MRI derived finite element human head model.
Estimates of steady state tissue temperatures were first obtained by evaluating the model under prescribed conditions with zero applied current density (no stimulation). The solution of the model with injected current density was then compared with the ‘no stimulation’ condition to evaluate relative tissue temperature increases. In addition for the 4 X 1 ring, we calculated tissue temperature increases for injected current density (Jn = 142.9 A/m2); corresponding to 13.58 mA total current. The aforementioned current density value has been reported to the minimum electrode current density at which brain lesions are observed in a tDCS rat model for stimulation duration greater than 10 minutes.
Conventional rectangular-pad stimulation results in no tissue temperature rise in the scalp or brain. The 4 X 1 ring (2 mA) configuration does not lead to a brain tissue temperature rise similar to conventional pad stimulation. However, negligible tissue temperature rises are observed at the scalp (<0.01 °C). For the 4 X 1 ring (13.58 mA), significant scalp temperature rise of 14.68 °C and a nominal brain temperature rise of 0.55 °C is predicted.